"It's a great tragedy for us," the director of JAMSTEC's Research Support Department, Hiroyasu Momma, told ABC Science Online.

The Kaiko's one-of-a-kind capability has made a unique contribution to marine science. It has brought undiscovered life-forms and previously inaccessible information about shifts in the Earth's crust to the surface. "The Kaiko is the only vehicle which can go to the deepest ocean floor in the world. For the time being, we have lost a method to survey more than 7,000 m deep," said Momma.

The Kaiko mini-sub first touched down on the bottom of the Pacific Ocean's Mariana Trench in 1995. At 11 km, the Trench is the deepest abyss on Earth, more than 2 km deeper than Mount Everest in the Hymalayas is high.

From the Mariana Trench, the Kaiko sampled 180 different kinds of microbes in the freezing darkness of its muddy floor, thriving at pressure 1,000 times greater than the surface air. It has also been used to help locate and recover sunken rockets and ships - but who rescues the rescuer?

On the recommendation of an expert panel, the official search has been called off, but Momma remains optimistic. "We don't believe that the vehicle lies on the bottom, because we received radio signals, which means that it surfaced," he told ABC Science Online via email. "There is a high possibility that the Kaiko is drifting below the surface. In this case, it is very difficult to recover the vehicle until it strikes a rock or reaches the shallow water."

The Kaiko is an unmanned Remotely Operated Vehicle (ROV) made of two parts: a 3-metre, 5-tonne mobile rover carrying four video cameras and two mechanical arms, tethered by a 250-metre secondary cable to a larger launcher carrying side-scan sonar. The entire ROV is connected by 12 km of towing, power and communication cables to the support ship at the surface.

In May, with a typhoon threatening the support ship Kairei at the surface, the main cable was hurriedly hauled in to retrieve the Kaiko. When the launcher was winched to the deck, however, the researchers discovered that its instrumented rover had been left behind.

It appears that the cable linking the rover to the launcher had snapped. "When we recovered the launcher we were surprised that the vehicle did not connect to the secondary cable," Momma said.

He believes that the Kaiko is indispensable and should be replaced, as researchers are keen to use its services. "We should maintain a capability to reach the full ocean depth because it is technically possible. The Kaiko makes [deep dives] easy and routine work."

"[If necessary] it will take between one and 1.5 years to build the new vehicle after we were funded. It will cost less than US$12 million and take four months for testing the new system."